This invention relates to a manufacturing method for making a protective coating on a machine-readable marking and to the product of the method. The method is particularly useful in a manufacturing method in which a machine-readable marking is produced on a glass or other nonporous workpiece.
U.S. Pat. Nos. 4,327,283 to P. M. Heyman et al. and 4,374,451 to W. R. Miller describe improved methods for assembling parts for a cathode-ray tube. The methods include providing at least one tube part, such as the glass faceplate panel, which has a unique machine-readable coded marking, such as a bar-code marking, on an external surface thereof. This marking is read one or more times by machine during the manufacture of the tube. Each time it is read, a control signal is generated in response to the reading, and then the signal is used to initiate a local process. In order for the reading to be reliable, the marking must be resonably clean at the time of each reading. Since the workpiece is subjected to various manufacturing processes, some of which involve chemicals and/or hostile environments, extraneous material which may interfere with the reading of the marking may deposit on the marking. It is, therefore, desirable to prevent such extraneous material from depositing on the marking and/or to provide a surface on the marking from which the extraneous material can be removed with an ordinary cleaning process without adversely affecting the readability of the marking.